When you travel in time, you can only go one way, forwards or backwards, but you can pick things up along the way. Young Xehanort picked up the incarnations of Ansem SOD and Xemnas from KH1 and KH2 respectively, while Master Xehanort picked up Xigbar and Saïx from a future point in time.
So now Master Xehanort has the time-travel power as well, as opposed to being brought there by someone else or being from the present time? So are we looking at a future MX who brought two of his own back with him to the gathering place? Why just those two? Why not the rest of his complete circle? And why even bother at all? If they're from the future than they already know that their attempt to turn Sora failed. This is why I said it's impossible for them to be from the future. They already stated that beyond the gathering, they knew absolutely nothing about what will happen. Unless the game or some other credible source says otherwise, this is what I will believe.
Again, I'm asking you: where are you getting all this detailed info from? Aren't you just extrapolating from all the vagueness and passing them off as if they were written in stone? I know it's fun to theorize and discuss possibilities but you might be taking things too far, given the vast majority of assumptions that have to be made in order for anything to make sense. It's true that DDD's plot was made purposely vague to retain the trademark "mystery" of the series and give them something to fill in later but maybe it would be better to not make up scenarios just to satisfy your desire to know all the answers.
That is not to say that you're entirely wrong, some of your posits could in fact happen as you say. But we can't know for sure until something more is revealed, and I'd rather not commit to any one idea completely until a few more holes in the narrative are filled to my satisfaction.
In closing, we know that AnsemSoD went back to confer the time travel power to a younger Xehanort, and said Xehanort used it to travel forward and bring with him various other "Xehanorts" to a specific place set up to catch the 13th. Nowhere, whether in the game or post-game interview w/ Nomura, was it stated that this power was also conferred to the older Xehanort. And we are led to believe that YX lost the time-travel power along with his memories once he was returned, so he couldn't have retained it for later use. So it stands to reason that only YX at that point of time had the power. This is what we know presently.
And so, the Xigbar and Saix we see in DDD can't be their complete selves: YX can't go farther into the future than the gathering place and pick up future incarnations of Braig and Isa and return with them for two reasons: 1) can only travel one way and 2) they would then have future knowledge that would negate the whole plan.
What ever happened to the crossroads of destiny? it was a pretty important place for the first few games and it hasn't been referenced in a long time. There's one in the Keyblade Graveyard but what happened to the one that Sora and Riku were on?
SYMBOLISM. Honestly, it probably wasn't anywhere near as important as the fans made it out to be.